Description

In the default implementation of the NSGA-II solutions with equal quality values are treated as non-dominated and are included multiple times in the Pareto front internally used by the algorithm.

Depending on the objective functions and used encoding solutions with equal quality values could overtake the Pareto fronts and the optimization is hampered. E.g., this happens when performing multi-objective symbolic regression due to solutions containing only one constant or variable.

Therefore, an option should be added to the algorithm that allows to specify how solutions of equal qualities should be treated (either dominated or non-dominated). However, the default behavior must be consistent with the default implementation of the NSGA-II.